Different methods for reducing the content of nitrogen oxides, hereinafter referred to as NOx, in exhaust gas from engines are known. One such method is Selective Catalytic Reduction, hereinafter referred to as SCR, through which NOx in the exhaust gas is converted into nitrogen N2 and water H2O by means of a catalyst. More particularly, ammonia or urea is added to the exhaust gas before this is fed through a SCR reactor, which contains a catalyst, for NOx decomposition. SCR reactors are well known within the art and will not be described in detail herein.
Depending on different factors such as the contents of the exhaust gas and the properties of the catalyst, there is a minimum temperature of the exhaust gas for proper NOx reduction/decomposition/conversion to take place. This minimum temperature is typically around 330° C. Depending on type and load of the engine, the temperature of the exhaust gas may vary within an interval T1-T2, where T1<330° C. To enable proper NOx reduction within the complete temperature interval, a heating unit for heating the exhaust gas above 330° C. may be provided. In prior art document WO 2008/135059 a system comprising a SCR reactor and a heating unit for increasing the exhaust gas temperature is described.
The provision of a heating unit for increasing the exhaust gas temperature is a simple and effective, but rather expensive, solution. Thus, within the art, room for improvement exists.